US5036325A  Doppler determination system for MTI radars  Google Patents
Doppler determination system for MTI radars Download PDFInfo
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 US5036325A US5036325A US07417358 US41735889A US5036325A US 5036325 A US5036325 A US 5036325A US 07417358 US07417358 US 07417358 US 41735889 A US41735889 A US 41735889A US 5036325 A US5036325 A US 5036325A
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 mti
 target
 prf
 return
 frequency
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 G—PHYSICS
 G01—MEASURING; TESTING
 G01S—RADIO DIRECTIONFINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCEDETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
 G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
 G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
 G01S13/50—Systems of measurement based on relative movement of target
 G01S13/52—Discriminating between fixed and moving objects or between objects moving at different speeds
 G01S13/522—Discriminating between fixed and moving objects or between objects moving at different speeds using transmissions of interrupted pulse modulated waves
 G01S13/524—Discriminating between fixed and moving objects or between objects moving at different speeds using transmissions of interrupted pulse modulated waves based upon the phase or frequency shift resulting from movement of objects, with reference to the transmitted signals, e.g. coherent MTI
 G01S13/526—Discriminating between fixed and moving objects or between objects moving at different speeds using transmissions of interrupted pulse modulated waves based upon the phase or frequency shift resulting from movement of objects, with reference to the transmitted signals, e.g. coherent MTI performing filtering on the whole spectrum without loss of range information, e.g. using delay line cancellers or comb filters
Abstract
Description
The present invention relates to moving target indicator (MTI) radars, and more particularly to a doppler determination system for MTI radars employing an amplitude comparison of odd and even MTI functions derived from the same signal returns to determine the doppler frequency of a target return.
In radar systems using simple MTI waveforms, no estimate of target doppler is obtained. This makes it impossible to determine unambiguous range rate from the use of multiple pulse repetition frequencies (PRFs) and makes it impossible to do accurate angle estimation because of lack of knowledge of target amplitude when adjacent beams have different waveforms. As a result of this lack of knowledge, a three dimensional radar design may be required which uses redundant transmissions of different waveforms at the same angular position in order to estimate target angle. Further, radars may employ scantoscan PRF changes for velocity visibility rather than beamtobeam or dwelltodwell because they could not otherwise determine angle from amplitude comparison between beams.
One class of 3 dimensional radar uses an antenna which rotates in azimuth while simultaneously generating a sequential scan of beams in elevation through either phase shifting or frequency scanning. The sequence of beams in elevation is called an elevation scan. The duration of an elevation scan is about the time that it takes the antenna to rotate through one azimuth beamwidth. The beams in elevation are spaced on the order of one elevation beamwidth. The elevation of the target is measured by comparing the relative amplitude from adjacent beams which straddle the target position. This technique is called sequential lobing and is discussed in "Introduction to Radar System," M. Skolnick, McGraw Hill, 1962, pages 165166. A simple estimate for the elevation angle is:
θ.sub.e =K(log P.sub.1 log P.sub.2)+θ.sub.o ( 1)
where P_{1} and P_{2} are the return single pulse amplitudes from the target on beams 1 and 2, respectively, k is a constant having to do with the beam spacing and beam width and θ_{o} is the angle at the crossover between the beams.
Often MTI waveforms are used on the lower beams of the elevation scan to suppress clutter. Higher beams are above the clutter and MTI is not required. Range gated pulse doppler is seldom used in a radar of this type because it takes more time than a simple three pulse MTI which would reduce significantly the elevation coverage.
One problem that has occurred on radars of this type is the inability to make an angle measurement when the target is straddled by two beams, one of which is employing an MTI waveform and the other of which uses a single pulse. The problem here is that the equivalent single pulse amplitude, P, from the MTI waveform is unknown.
It is also sometimes desirable to estimate the radar cross section (RCS) of the target. Normally this can be done from the knowledge of the sensitivity parameters of the radar coupled with the knowledge of the range to the target and the return amplitude. This is of the form
RCS=kP.sup.2 /R.sup.4 ( 2)
where P is the single pulse return amplitude from the target, K is a constant having to do with the sensitivity of the radar and R is the range to the target as measured by the radar.
When MTI is employed, the equivalent single pulse amplitude is unknown, and it has heretofore not been possible to estimate the target RCS using an MTI waveform.
Often, the doppler frequency from a target will be ambiguous. This can occur when the range of potential target doppler frequencies is larger than the pulse repetition frequency (PRF). If the ambiguous doppler frequency is known, two or more PRFs may be used to determine the unambiguous doppler frequency and hence the range rate of the target. This is known as PRF switching and is described in "Introduction to Airborne Radar," George Stimson, Hughes Aircraft Company, 1983, pages 364365. PRF switching is normally used with pulse doppler waveforms where an estimate of the ambiguous doppler may be made by comparing the amplitudes of the returns in adjacent doppler filters. This has not been useful to date for simple MTI waveforms because it was not possible to determine the doppler position of the return within the PRF interval.
It is therefore an object of the invention to provide a system for determining the doppler frequency (range rate) of the target return in a simple MTI radar system.
A further object of the invention is to provide a technique for estimating the equivalent single pulse amplitude of the MTI waveform and the elevation angle of a target detected by a 3dimensional radar system which generates a sequential scan of beams in elevation when the target is straddled by two beams, one of which employs an MTI waveform, and the other of which uses a single pulse waveform or which uses an MTI waveform with different PRF.
Another object of the invention is to provide a means for estimating the radar crosssection of a target detected by an MTI waveform.
In accordance with the invention, a system is disclosed for estimating the doppler frequency of the target return produced by a moving target indicator (MTI) radar. The system includes a first MTI circuit responsive to the target return for providing a first MTI return signal of amplitude A and characterized by a frequency response having odd symmetry about the center of the PRF interval. The system further includes a second MTI circuit responsive to the target return for providing a second MTI signal of amplitude B and characterized by a frequency response having even symmetry about the center of the PRF interval.
The system further comprises a means responsive to the first and second MTI signals for generating a ratio signal indicative of the ratio of the relative magnitudes of the first and second MTI signals. The ratio is then used to estimate the target doppler in accordance with the relationship PRF(1/2+(1/π)tan^{1} (2A/B).
The system is also advantageously employed in threedimensional radars for estimating the equivalent single pulse return amplitude of MTI waveforms, which can then be employed to estimate the target elevation angle and radar crosssection. The unambiguous range rate can also be determined.
These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which:
FIG. 1 is a simplified schematic block diagram of a doppler determination system embodying the invention.
FIGS. 2A and 2B are frequency response of the two MTI circuits employed in the system of FIG. 1.
FIG. 3 is a simplified block diagram illustrative of a digital system embodying the invention.
FIG. 4 is illustrative of beams of a 3dimensional radar which are scanned in elevation.
FIG. 5 is a simplified block diagram of a 3dimensional radar system employing the invention.
FIG. 6 is a functional flow diagram illustrating the utilization of the invention to determine the target doppler frequency, the equivalent single pulse amplitude, the target elevation angle, radar crosssection and unambiguous range rate.
The invention is illustrated in the simplified schematic block diagram of FIG. 1. Signals from the radar receiver 52 are applied to two MTI circuits. The first MTI circuit is a conventional double canceller circuit which comprises delay device 54, combiner device 56 for subtracting the receiver signal from the delayed receiver signal through delay device 54, delay device 58 and combining device 60 for subtracting the combined signal from combiner 56 from the delayed version of the combiner signal. The resultant signal is signal B. Each of the delay devices 54 and 58 introduces a 1/PRF time delay.
The second MTI circuit comprises the delay devices 62 and 64 which collectively delay the receiver signal by 2/PRF, and the combiner device 66 for subtracting the receiver signal from the delayed signal. The resultant combiner 66 output is signal A. This second canceller circuit resembles a single canceller circuit, except that it uses two times the delay of an interpulse period, i.e., 2×(1/PRF).
The two signals A and B are applied to an amplitude comparison circuit 70 which generates the ratio of the amplitudes of signal A to signal B, including polarity. This ratio signal A/B is further applied to a circuit (or computer software) 72 for generating the estimated value of the doppler frequency, f_{d}, from the ratio signal.
Exemplary waveforms illustrative of the frequency responses of signals A and B are shown in FIGS. 2A and 2B, respectively. As shown in FIGS. 2A and 2B, the response of signal A has odd symmetry about the center of the PRF interval, while the response of signal B has even symmetry about the center of the PRF interval. The signals A and B are defined by equations 3 and 4.
A=2 sin 2π((f.sub.d PRF/2)/PRF) (3)
B=4 (1+cos 2π((f.sub.d (PRF/2)/PRF) (4)
The ratio of the signals A/B is given by equations 5 and 6. ##EQU1##
The estimated doppler frequency f_{d} is given by equation 7.
f.sub.d =PRF(1/2+(1/π)tan.sup.1 (2A/B)) (7)
A digital implementation 100 of the system is illustrated in FIG. 3. Here the signals from the radar receiver 102 are fed to mixers 104 and 106 for mixing with a local oscillator signal (mixer 104) and a version of the local oscillator signal which has been delayed by 90° (mixer 106). This yields inphase I and quadrature signals Q which are applied to respective analogtodigital converter (ADC) devices 108 and 110. The resultant digitized inphase I and quadrature Q signals are applied to the digital processor 112 for processing to develop an estimate of the doppler frequency f_{d} in accordance with the invention.
The processor 12 processes the I and Q signals to obtain the inphase and quadrature components of the A and B signals, i.e., I_{A}, I_{B}, Q_{A} and Q_{B}.
I.sub.B =I.sub.1 2 I.sub.2 +I.sub.3 (8)
Q.sub.B =Q.sub.1 2Q.sub.3 +Q.sub.3 (9)
I.sub.A =I.sub.1 I.sub.3 (10)
Q.sub.A =Q.sub.1 Q.sub.3 (11)
where I_{1} and Q_{1} are the respective inphase and quadrature components of the return signal from the first transmitted pulse, delayed by 2/PRF to be time coincident with the return from the third transmitted pulse. Similarly, I_{2} and Q_{2} are the respective inphase and quadrature components of the return signal from the second transmitted pulse, delayed by 1/PRF to be time coincident with the return from the third transmitted pulse, and I_{3} and Q_{3} are the respective inphase and quadrature components of the return signal from the third transmitted pulse.
The signals A and B are given by equations 12 and 13.
A=I.sub.A +jQ.sub.A (12)
B=I.sub.B +jQ.sub.B (13)
A is orthogonal to B since A is an odd function and B is an even function. The ratio A/B is equivalent to jA·B/B^{2}, where j appears because of the orthogonality of the signals A and B. The ratio can also be written as
A/B=j(jI.sub.A Q.sub.B +jI.sub.B +I.sub.A I.sub.B Q.sub.A Q.sub.B)/(I.sup.2.sub.B +Q.sup.2.sub.B) (14)
Because I_{B} I_{A} =Q_{B} Q_{A} =0 since signals A and B are orthogonal, equation 14 can be rewritten as equation 15.
A/B=(I.sub.A Q.sub.B +I.sub.B Q.sub.A)/(I.sup.2.sub.B +Q.sup.2.sub.B) (15)
The estimated doppler frequency f_{d} can then be determined using the relationship of equation 7. The determination may be readily implemented with a PROM lookup table.
As described above, one problem that has occurred on 3dimensional radars is the inability to make an angle measurement when the target is straddled by two beams, one of which is employing an MTI waveform, and the other of which uses a single pulse, since the equivalent single pulse amplitude P from the MTI waveform is unknown. The beams of a simple 3dimensional radar are illustrated in FIG. 4. By using the estimated doppler frequency obtained in accordance with the invention, an estimate of the single pulse amplitude from the MTI waveform can be made. This in turn allows a direct comparison to be made between the returns from the two beams using the sequential lobing technique described above to make an estimate of the elevation angle.
FIG. 5 illustrates a simplified block diagram of a threedimensional radar system 150, comprising transmitter 152 and receiver 156 which are connected to the system antenna 164 via the duplex 154. The radar receiver output signals are fed to the respective MTI circuits 158 and 160. The MTI circuit 158 is an even function MTI circuit which, for example, may be implemented as the double canceller circuit described with respect to FIG. 1. The MTI circuit 160 is an odd function MTI circuit which, for example, may be implemented as the single canceller circuit using a delay of 2/PRF as described above with respect to FIG. 1. The respective MTI circuit outputs of amplitudes B and A are fed to the processor 162. Depending on the application, the processor can comprise circuitry or (in a digital implementation) software for performing the functions described below.
The signals A and B have each been normalized to the single pulse response P which would have been obtained with a non MTI waveform in the absence of clutter return. This normalization is inherent if the delay has unity gain, as it would have for a digital implementation. Thus,
B=4(1+COS2π(f.sub.d /PRF1/2))P. (16)
P can be solved for a function of A and B. Substituting equation 5 into equation 16, the relationship of equation 17 is obtained.
B=4(1+COS2(TAN.sup.1 (2A/B))P (17)
Because COS(2θ)=2COS^{2} θ1, equation 17 can be rewritten as follows:
B=4(2COS.sup.2 (TAN.sup.1 (2A/B))P (18)
Because cos(tan^{1} (2A/B))=B/(B^{2} +4A^{2})^{1/2}, it follows that
B=(8B.sup.2 /(B.sup.2 +4A.sup.2))P, and (19)
P=B(B.sup.2 +4A.sup.2)/8B.sup.2 (20)
Thus, an estimate of the equivalent single pulse amplitude P is obtained from the signal magnitudes A and B after having rejected the clutter return.
The simple estimator of equation (1) can be used to estimate the elevation angle θ_{e}.
The invention may also be used where the target is straddled by two MTI beams with different PRFs, i.e., the PRF switching technique described above, since in each case an estimate can be made of the equivalent single pulse amplitude. Since the invention provides an estimate of the doppler position (f_{d}) of the return within the PRF interval, PRF switching can be employed with simple waveforms. With the PRF switching technique using two PRFs having a PRF difference s, the observed doppler frequency will not change if the target return is in the first PRF interval, will change by s if in the second PRF interval, by 2s if the third PRF interval, and so on. Thus, the unambiguous doppler frequency will be the measured ambiguous doppler frequency plus N times the PRF on which that measurement was made.
f.sub.d =(f.sub.d)+(N) (PRF) (21)
N is the multiple of s by which the measurement shifted when the PRF changed.
Once the unambiguous doppler frequency has been determined, the target range rate may be determined by the following relationship.
Range Rate=(λf.sub.d)/2 (22)
FIG. 6 is a functional flow diagram illustrating the utilization of the invention to determine the target doppler frequency, the equivalent single pulse amplitude, the target elevation angle, radar crosssection and unambiguous range rate. This corresponds to functions which may be carried out by the digital implementation of FIG. 3. At steps 202 and 204, the inphase and quadrature components of the received signal are converted from analog form to digital signals (corresponding to the functions of A/D converters 108 and 110 of FIG. 3).
At steps 206 and 208, the digital forms of the inphase and quadrature components are subjected to digital delays to form the odd and even function MTI waveforms. The signals B and A (as described above) can then be formed at steps 210 and 212.
The signal B represents the output from the double canceller main MTI target detection channel, and at step 214 the value of the signal B is compared to a noise threshold value to distinguish a target from noise. When the value of B exceeds the noise threshold, a target is detected, enabling the gates at step 216 which puts into operation several operations. One operation is to estimate the ambiguous target doppler frequency (step 218) using the relationship of equation 7. When the PRF switching technique described above is employed, the unambiguous range rate can be established by first determining the unambiguous target doppler frequency in the manner described above regarding equation 21, and from that doppler frequency the unambiguous range rate can be determined using the relationship of equation 22. Thus, the ambiguous doppler frequency is first determined for the PRF1 and stored in memory (step 220). Next, the ambiguous target doppler frequency at the PRF2 is determined and stored in memory (step 222). From the stored values, the unambiguous target doppler frequency and the range rate are then determined (step 224).
Once a target is detected, the equivalent single pulse amplitude P of the MTI waveform can be estimated (step 226) via the relationship of equation 20. From the estimate of P and the target range (determined at step 228 by wellknown techniques), the radar cross section (RCS) is determined via the relationship of equation 2.
The estimate of the equivalent single pulse amplitude P of the MTI waveform can further be employed in a threedimensional radar to estimate the elevation angle of the target, where the target is straddled by beams N and N+1. First, the amplitude P of the MTI waveform, say from beam N, is estimated and stored (at step 232). Next, the single pulse amplitude of the next beam N+1 (or the equivalent single pulse amplitude if beam N+1 also employs an MTI waveform) is determined and stored (at step 234). The target elevation angle is then estimated (at step 236) via the relationship of equation 1. It is to be understood that the invention is not limited to estimating the target elevation angle, but is useful generally in estimating the angle position of a target straddled by two beams, at least one of which employs an MTI waveform.
It is understood that the abovedescribed embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope of the invention.
Claims (19)
f.sub.d =PRF (1/2+(1/π)tan.sup.1 (2A/B)).
f.sub.d =(f.sub.D (PRF1))+N(PRF 1)
P=B(B.sup.2 +4A.sup.2)/8B.sup.2.
f.sub.d =PRF (1/2+(1/π)tan.sup.1 (2A/B)).
f.sub.d =(f.sub.D (PRF 1))+N(PRF 1)
P=B(B.sup.2 +4A.sup.2)/8B.sup.2.
P=B(B.sup.2 +4A.sup.2)/8B.sup.2.
RCS=k P.sup.2 /R.sup.4
θ=k(logP1logP)+θ.sub.o,
θ=k(logP2logP1)+θ.sub.o,
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US6411249B1 (en) *  20000719  20020625  Northrop Grumman Corporation  Apparatus and method for the monopulse linking of frequency agile emitter pulses intercepted in on single interferometer baseline 
Citations (12)
Publication number  Priority date  Publication date  Assignee  Title 

US3914763A (en) *  19730830  19751021  United Aircraft Corp  Hybrid filter employing digital techniques and analog components and amti radar employing same 
US4041488A (en) *  19760419  19770809  The United States Of America As Represented By The Department Of The Army  Doppler radar system 
US4093950A (en) *  19770516  19780606  The United States Of America As Represented By The Secretary Of The Navy  Motioncompensation arrangements for MTI radars 
US4093951A (en) *  19770516  19780606  The United States Of America As Represented By The Secretary Of The Navy  Compensation for simultaneous platform motion and antenna scanning in MTI radars 
US4222049A (en) *  19770624  19800909  ThomsonCsf  Circuit arrangement for eliminating fixed echoes in a pulse 
US4513287A (en) *  19810828  19850423  ThomsonCsf  Device for the elimination of nth trace moving echoes and interference echoes in a radar 
US4542382A (en) *  19810702  19850917  Hollandse Signaalapparaten B.V.  Search radar apparatus 
US4630052A (en) *  19830504  19861216  SeleniaIndustrie Elettrotechniche Associate S.P.A.  Suppressor of secondtimearound clutter echoes for MTI pulse radar provided with power oscillator 
US4654665A (en) *  19830721  19870331  Nec Corporation  Radar system 
US4684950A (en) *  19840720  19870804  Long Maurice W  Methods of and circuits for suppressing doppler radar clutter 
US4713664A (en) *  19850524  19871215  Westinghouse Electric Corp.  Point clutter threshold determination for radar systems 
US4914442A (en) *  19890130  19900403  The United States Of America As Represented By The Secretary Of The Navy  Adaptive MTI target preservation 
Patent Citations (12)
Publication number  Priority date  Publication date  Assignee  Title 

US3914763A (en) *  19730830  19751021  United Aircraft Corp  Hybrid filter employing digital techniques and analog components and amti radar employing same 
US4041488A (en) *  19760419  19770809  The United States Of America As Represented By The Department Of The Army  Doppler radar system 
US4093950A (en) *  19770516  19780606  The United States Of America As Represented By The Secretary Of The Navy  Motioncompensation arrangements for MTI radars 
US4093951A (en) *  19770516  19780606  The United States Of America As Represented By The Secretary Of The Navy  Compensation for simultaneous platform motion and antenna scanning in MTI radars 
US4222049A (en) *  19770624  19800909  ThomsonCsf  Circuit arrangement for eliminating fixed echoes in a pulse 
US4542382A (en) *  19810702  19850917  Hollandse Signaalapparaten B.V.  Search radar apparatus 
US4513287A (en) *  19810828  19850423  ThomsonCsf  Device for the elimination of nth trace moving echoes and interference echoes in a radar 
US4630052A (en) *  19830504  19861216  SeleniaIndustrie Elettrotechniche Associate S.P.A.  Suppressor of secondtimearound clutter echoes for MTI pulse radar provided with power oscillator 
US4654665A (en) *  19830721  19870331  Nec Corporation  Radar system 
US4684950A (en) *  19840720  19870804  Long Maurice W  Methods of and circuits for suppressing doppler radar clutter 
US4713664A (en) *  19850524  19871215  Westinghouse Electric Corp.  Point clutter threshold determination for radar systems 
US4914442A (en) *  19890130  19900403  The United States Of America As Represented By The Secretary Of The Navy  Adaptive MTI target preservation 
Cited By (1)
Publication number  Priority date  Publication date  Assignee  Title 

US6411249B1 (en) *  20000719  20020625  Northrop Grumman Corporation  Apparatus and method for the monopulse linking of frequency agile emitter pulses intercepted in on single interferometer baseline 
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